They're intensely interested in health care, they turn out in droves for town hall meetings, they're vocal - and they vote.

That is why millions of American seniors - like Richard W. Kirby, 71, a retired attorney who lives in the Rossmoor senior community in Walnut Creek - may become the muscle vote of the 2012 elections, political analysts say.

Kirby, at a town hall meeting in his neighborhood this week that was led by Rep. John Garamendi, D-Walnut Grove (Sacramento County), said he'll be watching the political arm-wrestling over House Budget Committee Chairman Rep. Paul Ryan's 2012 budget proposals. Kirby's chief concern: the Republican plan for dramatic overhaul of Medicare, the popular entitlement health program.

"This is a budget designed to make the Tea Party people happy," said Kirby, a Democrat. "They'll have to rethink their ideas."

His wife, Sally Kirby, who is a registered Republican, shook her head, saying her own party's proposals are "so embarrassing" that she has "been thinking of re-registering."

Preview of 2012 vote

With the 2012 election just 18 months away, the Kirbys were among the opinionated crowd at Rossmoor that was a preview of how voters in the 50-plus electorate - nearly half of all Americans who go to the polls - could call the shots as Democrats and Republicans battle over how to slash the looming federal deficit.

The GOP has been on defense with older voters since Ryan's budget plan passed the House on April 15 with the support of all but four Republicans. Since then, Republicans have faced scathing criticism in public settings from seniors on both sides of the aisle, who say they have deep concerns about turning Medicare into a program where the government subsidizes future retirees through vouchers and private insurance.

GOP freshmen House members held a news conference Tuesday asking Democrats to "hit the reset button" on the discussion - a plea Democrats are ignoring.

President Obama wants to keep Medicare a government program but empower a panel of experts to order cuts if spending exceeds a certain target. His latest proposal would strengthen cost curbs in the new health care overhaul.

Line of attack

Garamendi, whose 10th Congressional District stretches from Antioch and Fairfield to Walnut Creek, dramatized the effectiveness of his party's line of political attack against the GOP plan in an address to about 100 seniors in the town hall session Monday.

"The thing that really troubles me is where they want to go in the future, to terminate Medicare," the former California insurance commissioner and lieutenant governor told them.

"If you're 55 years or younger, you will not have Medicare when you turn 65. It's gone. You'll be given a voucher," he warned, to buy insurance in the free market. "Good luck with that."

But Tom Del Beccaro, chairman of the state GOP, echoed his party's recent response to criticism of Wisconsin Rep. Ryan's plan - charging that Democrats are clearly more interested in "scaring" voters than offering serious solutions to the debt crisis.

"By 2025, there are estimates that Medicare and Social Security combined will take up all of our tax revenues," said Del Beccaro, who called Ryan's debt-reduction plan "thoughtful."

He argued that "a bankrupt government is not a compassionate government."

But the growing political clout of seniors is dramatized by government census data released last month showing that people between 55 and 64 years old are the fastest-growing group since 2000, as a proportion of the U.S. population.

AARP opposes GOP plan

Their increasing clout is one reason why the proposal by Ryan has become the subject of a tough national ad campaign by AARP, one of the nation's largest lobbying groups. The organization for Americans over 50 criticized what it called "harmful cuts" to Medicare and Social Security favored by Republicans - prompting an adviser to Ryan's political action committee to dismiss the 35 million-member organization as a "left-leaning pressure group."

Barbara O'Connor, a member of the AARP board of directors, said the nonpartisan group is raising the issue to reflect the widespread concern and demands for "a substantive debate" about Medicare.

"What is needed is a real coherent discussion of what we want America to look like," she said. O'Connor added that polls and public sessions like the one held by Garamendi this week show that "people are not likely to lie down quietly" should the Republicans try to substantially change the Medicare system.

Political libertarian talk show host Patrick Dorinson said both sides of the aisle are trying to calculate how to seize the issue to best effect. The so-called "Medi-scare" tactics, he said, are "all about the Democrats figuring: How do we get back the House? They want to rebuild their coalition."

Between Democrats and Republicans, he said, "We had six months of talking about issues, and now we'll start talking about who's the bigger skunk."

But Rossmoor resident Florence Kleinfield, 76, one of the many at Garamendi's town hall who asked sharply detailed questions about proposed budget cuts, said Republicans can try all they want to turn Medicare into a voucher program.

"It's not even worth discussing it," she said dismissively of the GOP proposals. "They're not going to touch the third rail."

GOP Medicare plan

Rep. Paul Ryan's budget plan, approved by the House last month, included this plan for Medicare:

Voucher-like program: Future Medicare beneficiaries - those currently 54 years old and younger - would purchase health insurance instead of having the government pay doctor and hospital bills directly.